Generally, magnetic flow meters are used to measure volume rates of electrically conductive fluids which present difficult handling problems. Example of such fluids include corrosive acids, sewerage, detergents, and the like. These meters typically include a short section of meter pipe placed in the fluid pipeline. Electrodes are disposed on opposite sides of this tube and in electrical contact with the conductive fluid. Volume rates of the fluids are measured by providing a uniformly distributed magnetic field which is perpendicular to the longitudinal axis of the flowing fluid and at right angles to the electrodes. An instantaneous voltage value is established at the electrodes, which is perpendicular to both the velocity of the fluid and the flux linkages of the magnetic field, and is proportional to the rate of flow of the fluid.
Because of the corrosive and abrasive nature of the fluids, the short meter pipe section is often made of a high purity aluminum oxide ceramic which has excellent corrosive properties and good mechanical stability and strength. A problem associated with using a ceramic pipe section or tube, however, is the inability to seal the electrode to the ceramic given the harsh operating conditions.
To overcome this problem, the present inventor designed an electrode sealing assembly which includes an electrode planted in an interior cavity formed along the inner surface of the meter tube. An electrode stem is connected to the electrode and extends through the walls of the meter tube for external electrical connection. An interior electrode seal is established by placing a multilayer polymer gasket between the electrode and the inner surface of the meter tube. One layer of gasket material is a thin layer of polymeric material, such as perfluoroalhoxyethylene (PFA), which is melted to intimately fill surface variations in the ceramic flow tube seal area. A second thicker teflon gasket layer, such as polytetrafluoroethylene (PTFE), is placed between the thinner teflon gasket layer and the electrode to provide a resilient seal which protects the integrity of the melted-in seal. The electrode is biased against the multi-layer gasket and the interior wall of the ceramic tube by a Belleville washer assembly. The washer assembly, together with the pressure of the fluid flowing in the pipeline, ensures the integrity of the seal despite any future creeping in the gasket due to aging or temperature changes. This seal assembly is described in greater detail in U.S. Pat. No. 4,773,275 issued Sep. 27, 1988 and entitled "Seal For Ceramic Flow Tube." This patent is hereby incorporated by reference.
For a ceramic flow tube having an inner diameter of one half inch or greater, this electrode seal assembly works exceptionally well. A disadvantage of this technique, however, is that it is difficult and expensive to precisely machine the interior cavity of the small ceramic tubes used to house the electrode. Further, it is not practical to use this electrode seal assembly technique to seal electrodes to fractional size ceramic flow tubes having diameters less than one half inch because it becomes an extremely difficult task to make a precise counterbore along the interior wall of the smaller meter pipe. It is also difficult to inspect the interior bored-out cavity to insure its proper tolerances. The process of heating the electrode assembly to melt and bond the polymeric gasket material is also an expensive and time consuming process.